It becomes extremely hard to find out availablity of fight as well as miles required if you want to redeem your SkyMiles on Delta.
Here is a trick that may help.
In place of trying to search a round-trip, just look for one way trip and try to get a 5 week calendar ; that will allow you to see across several days with options (see ATL-> LAX below).
While this may not look like a great thing to start with: consider an airport that Delta may not be flying to at all or if you are trying to get a peak period ticketing: See ATL—>BOM below where the flights are not availalble for redumption for some days or price shoot up to about 3 times: